The invention relates to active electronic filters and their use in medical diagnostic equipment.
Active electronic filters are in wide use today because solid state devices have made the required, active, switching functions practical and because high selectivity filters can be designed from actively switched networks of resistors and capacitors without cumbersome inductors. One type of active electronic filter is an N-path filter.
N-path filters have a number N of parallel resistive-capacitive filter sections and a switching arrangement to connect each filter section periodically into an input signal path through the filter. Each filter section has an identical transfer function. This has required matching the charging characteristics of the capacitors in the filter sections. Matching the charging characteristics of a set of capacitors for each filter is a tedious, time-consuming, costly, individual assembly operation for each filter. Additionally, mismatches in the capacitors reduce the depth of the rejection notch at the filter frequency. A way to reduce the effect of variations in the charging characteristics of the capacitors sufficiently to allow off-the-shelf capacitors to be used in an N-path filter without substantial matching would thus make N-path filters much easier to make, particularly if the technique used other off-the-shelf components which did not require matching or other arduous assembly techniques.
Except for their difficult matching problem, the precision of N-path filters would be particularly useful to achieve the medical-quality precision required in electronic medical diagnostic equipment. The body functions to be diagnosed with such equipment are analog. Transducing the analog body function into an analog signal for processing into an analog output is therefore a desirably direct approach, but precision filters are required to process the analog signal precisely. Electrocardiographs and electroencephalographs operated from line-frequency power exemplify the need for precision filters in such analog medical diagnostic equipment.
The frequencies of the functions to be diagnosed with such devices include the usual power line frequencies, 50-60 Hz. A filter to eliminate the narrowest possible notch of frequencies about the line frequency to eliminate power line frequency interference from the diagnostic signal while retaining as much as possible of the rest of the diagnostic signal bandwidth is therefore desired. N-path filters can filter out narrow notches, i.e. have a high Q factor, and therefore would be desirable in such diagnostic devices except for the difficult matching problem in making them.